Information and multimedia services today are predominantly published and distributed in a digital format rather than an analog format. The reasons for the shift to digital formats include the ease and economy at which digital content can be distributed. However, digital content providers face a constant threat of unauthorized copying and distribution of their digital content.
Techniques have been developed to help protect digital content from unauthorized or pirated use. The techniques include systems for encrypting and/or authenticating the digital content. One example of authentication is the use of digital watermarks. Digital watermarking is the process of inserting one or more sequence of bits, called watermarks, into a digital file or signal, referred to as the host signal. The watermarks may be used to carry information about the host signal, including authorship and ownership. Moreover, the watermarks or lack thereof may be used for authenticating and/or tamper proofing purposes.
A digital watermark may be perceptible or imperceptible. A perceptible watermark is one that alters the host file or signal such that the watermark itself is perceptible to a consumer. For example, the digital watermark may add a copyright notice onto an image that is visible to the consumer. However, the utility of a perceptible watermark is limited.
For example, the visibility of the watermark makes the location of it known to potential attackers which makes it easier for the attackers to remove it. Also, in some applications such as imaging and video, it is undesirable to mar an image or other digital content with a perceptible watermark.
In contrast, because an invisible watermark is imperceptible, its location within the host file or signal is unknown, which in turn makes it harder for someone to remove or alter the watermark. However, an invisible watermark is not immune to attacks, deliberate or otherwise. For example, if a watermark is casually embedded into the host signal it can be distorted or even completely destroyed by simple operations such as cropping or filtering.
“Robust watermarks” have been developed that are capable of withstanding a reasonable degree of attacks. In general, two types of techniques, “spread spectrum” and “diversity based,” have been used to develop robust watermarking.
A spread spectrum technique is the insertion of a pseudo-randomly generated sequence of bits, i.e. the watermark, in a host file or signal in either a spatial domain or in a transform domain, e.g., discrete cosine transform (DCT) coefficients or wavelet transform coefficients. Later a digital file or signal may be tested for the presence of the watermark by extracting the bits from the digital signal that correspond to the expected location of the watermark. If the extracted bits correlate with the known or reference watermark sequence to a sufficient degree, that correlation would indicate the presence of the original watermark and the authentication of the digital signal as the host signal.
Although the spread spectrum technique has been effective against some narrow band interference, it has not proven effective for extraction of long watermark sequences due to the high bandwidth requirements. Other known drawbacks associated with the spread spectrum technique include vulnerability to near-far problems and inflexibility to statistical variations in the signal.
A diversity based technique has been proposed to overcome some of the drawbacks of the spread spectrum technique. In general, a diversity based technique involves embedding and extracting multiple copies of two types of watermarks, reference watermarks and robust watermarks. The copies of a reference watermark are used to estimate the robust watermark's reliability. Specifically, the multiple copies of the reference watermark are used to estimate the bit error probabilities or distortion of the extracted reference watermark bits and then the multiple copies of the extracted robust watermark are combined, through linear weighting and the estimated bit error probabilities, to find an overall estimate of the original robust watermark. A high enough correlation value between the original undistorted robust watermark and the overall estimate of the original robust watermark would indicate the presence of the original robust watermark and the authentication of the digital file or signal.
The reliance on two types of watermarks adds to the payload of the signal or increases the size of the file. Also, the diversity based technique makes the assumption that the bit error probabilities for the copies of the reference watermark is the same as the bit error probabilities for the copies of the robust watermark. This assumption may be problematic when localized distortions are introduced to the digital signal that affects one type of watermark more than the other. Furthermore, if a localized attack destroys the reference watermarks but leaves the robust watermark unaltered, the prior art robust watermarking schemes will fail to recover the robust watermark. Hence the efficiency of prior art robust watermarking schemes is not in proportion to the payload embedded within the host signal.
Given the above described limitations and problems, it may be advantageous to provide an improved method, apparatus, and computer program product for embedding and extracting digital watermarks in and out of digital files and signals.